List the three criteria early chemists used to classify a compound as a base.

One of the hallmarks of bases is their bitter taste, which is quite distinct from the sour taste of acids.
Early chemists relied on this characteristic to classify compounds. While our taste buds are adept at recognizing this flavor, it's important to note that tasting chemicals in a lab setting is incredibly unsafe. Today, we use other means, such as pH indicators or electronic pH meters, to determine the nature of a substance.
Yet, the unique bitter taste remains an enduring historical footnote in the classification of substances as bases. It serves as an example of how early scientific methods have evolved over time, leading to modern safety practices around chemical identification.

The slippery texture of bases is another classical characteristic noted by early chemists.
This distinct quality comes from the reaction of the base with the oils on our skin, often resulting in a soapy texture. In more chemical terms, many bases are alkaline and can saponify certain fatty acids, a process similar to how soap is made.
While this physical property played a key role in historical base identification, contemporary scientists use more precise techniques such as conductivity tests. This ensures that there is no direct skin contact with potentially hazardous chemicals, prioritizing safety while still acknowledging the past criteria used for classifying bases.

The neutralization reaction is a definitive test for the presence of a base when it's mixed with an acid.
Through this reaction, a base 'neutralizes' an acid, forming water and a salt. This process can be represented by the equation \( \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water} \).
Historically, witnessing a change in temperature, formation of bubbles, or a color shift provided concrete evidence of this chemical reaction. Today's scientists use this foundational understanding in complex applications like titrations, where they can precisely measure the concentration of an acid or base in a solution. At its core, the neutralization reaction remains a quintessential property used to identify bases and illustrates a vital interactive principle in chemistry.